This invention relates to electrolytic coloring processes for anodized aluminum surfaces.
The process of coloring an aluminum or aluminum alloy workpiece by electrolytic means has been widely used and described in the literature, which discloses the basic process as well as numerous variations in both materials and operating conditions. The most common procedures are done subsequent to anodization and involve the use of one or more nickel salts in an acidic electrolyte solution using alternating current. The most common nickel salts are nickel sulfate, acetate, and chloride.
In spite of the long history and wide use of this process, the mechanism by which coloring is achieved is not well understood. Until recently, for example, both the nickel salt concentration and the operating temperature were maintained at low levels, since no benefit was known to occur at higher levels to justify the increased cost, and the higher levels were thought to be detrimental to the throwing power of the bath, i.e., its ability to produce a uniform color over the entire surface of the workpiece. A way of improving the throwing power is reported in commonly assigned U.S. Pat. No. 4,431,489 (Baker et al., Feb. 14, 1984), whereby nickel sulfamate is used as the predominant nickel component of the bath.